Battery overcharging is one of the most destructive events that can occur in the life of a battery, an event that leads to undesirable reactions occurring within the battery and the generation of excessive heat. If these reactions go unchecked, the increase in heat generation quickly reaches the point at which more heat is being generated than can be withdrawn, leading to the condition commonly referred to as thermal runaway. At this point, the amount of heat being generated is great enough to lead to the combustion of the battery as well as materials in proximity to the battery.
To reduce the risk of thermal runaway, most rechargeable cells include one or more built-in safety mechanisms that are designed to automatically take effect during an abusive situation (e.g., overcharging, internal short circuit, physical abuse, manufacturing defects, etc.). For example, a conventional cell will often include an internal positive temperature coefficient (PTC) current limiting device, a current interrupt device (CID), and a venting mechanism, the venting mechanism designed to rupture at high pressures and provide a pathway for cell contents to escape. The PTC element is designed to exhibit a very high impedance when the current density exceeds a predetermined level while the CID is designed to break the electrical connection between the electrode assembly and the cell terminal if the pressure within the cell exceeds a predetermined level.
While individual cells may include one or more built-in safety mechanisms, as noted above, these safety mechanisms are not always effective when the cell is one of a large group of cells. For example, the CID within a cell typically has a relatively low voltage rating and, as a result, may be subject to arcing and fire when it attempts to open in a high voltage battery pack. Accordingly, many conventional rechargeable battery packs may include one or more overcharge protection systems at the system level, each of which is designed to prevent the battery or batteries within a battery pack from being overcharged. These systems can be divided into those associated with the battery pack itself, and those associated with the charger/charging circuit. On the battery side, usually one or more voltage sensing circuits are used to monitor the condition of the batteries, either individually or as a group of cells. When these circuits sense overcharging, they disrupt the connection between the battery pack and the charging system, typically by opening the contactor or pair of contactors that couple the battery terminals to the charging circuit. The use of a pair of contactors, one coupled to either terminal, versus a single contactor, provides an additional level of protection. On the charger side, sensing circuits are used to monitor the load, i.e., the battery pack, coupled to the charging circuit. When the charging system determines that overcharging is occurring, or about to occur, the charging system is designed to terminate charging.
While one or more levels of overcharge protection are included in most systems utilizing rechargeable batteries, there is still a risk of an overcharging event occurring, for example due to the failure of both a charging circuit and a contactor. If such a failure were to occur in a system utilizing a large battery pack, overcharging could lead to all of the cells within the pack undergoing nearly simultaneous thermal runaway. While the collateral damage of such an event could be huge, if it were to occur in a safety sensitive application such as an electric vehicle, the consequences could be catastrophic. Accordingly, although the prior art discloses various systems that provide protection from an overcharging event, an additional layer of protection that is independent of the contactors and the charging circuit is desirable. The present invention provides such an additional layer of protection.